Merge two planes one above the other
Hi Zim in this case its best to use the ShrinkWrap or KnifeProject. The 1st one is more simple to use so thats what we try. I prepared the scene looking like this:



You could see the road above the terrain so now its good to make the road subdivided (preferably more that the terrain) for ShrinkWrap being possible so use Subdivide to achieve this result:



Its enough to acomplish what we want so now the modifier takes place. Add it and as a target choose the terrain object (in my case its "Plane"). Now use the settings as below (GREEN). As a MODE use the Project and dont forgot to check the Negative projection (in our case we have road above the surface so its negative). Also dont forget to set the Offset to about 0.01 (lets try lowest value possible) to avoid overlapping faces.



Everything is done so hope it helps Buddy.

Sincerely, Jan